The invention concerns a method and device for magnetic resonance imaging which is particularly suitable for sodium imaging in clinical applications.
The application of sodium MRI in diagnostic radiology and clinical research has been limited due to the technical difficulty of imaging this quadrupolar nucleus. The inherent low concentration of tissue sodium (intra- and extra-cellular concentrations of approximately 10 and 140 mM respectively in the brain1), along with very short transverse relaxation times (3 to 8 ms)2 make this a challenging nucleus to image with adequate signal-to-noise and scan time for clinical use. Despite these constraints, a number of clinical uses of sodium imaging have been reported. The clinical areas most likely to benefit from this technique are in the diagnosis and management of acute stroke1, myocardial infarction,3 skeletal muscle disease4 and osteoarthritis and cartilage degeneration.5 As sodium imaging has been shown to be a better indicator of ischemic tissue compared with diffusion weighted imaging (DWI) in animal models of stroke6 there is potential for fast sodium imaging methods to replace the requirement of high-end echoplanar MR scanners capable of DWI for the clinical assessment of acute stroke patients.
Projection imaging (PI) methods7 and conventional 3D fast gradient echo sequences (FLASH)8 have previously been adapted to image sodium. Unfortunately, these early PI methods have suffered from inefficient k-space sampling strategies, and gradient echo methods from excessive T2 weighting. Three-dimensional twisted projection-imaging (TPI) sequences with extremely short echo times (TE˜0.4 ms) have recently been shown to produce sodium images of diagnostic quality.9 This approach however does not utilize the potential of the scheme to minimize the noise variance of a sodium image through the uniform sampling of k-space. There is therefore a need for demonstrating how the original concept that resulted in development of the 3D TPI methodology can be used to produce three-dimensional uniform sampling density (USD) k-space trajectories, further improving the efficiency of 3D projection imaging for sodium.
In view of the above aspects of prior art it is the underlying purpose of the present invention to develop a method and device for magnetic resonance imaging which is particularly advantageous for applications involving sodium nuclei, which effects efficient, uniform sampling density with adequate signal to noise ratios for the extracted signals and with scan times which are acceptable for clinical applications.